The importance of the sensor network technique is increasing which has a radio communication function as an interface between a real space and a virtual space in a computer graphics in the ubiquitous society. In the sensor network, many radio sensor nodes carry out sensing operations to transmit the obtained data, respectively. The transmitted data are collected by a server. Thus, various situations in a wide range such as traffic jam and environments can be grasped.
As the sensor node (sensor terminal), there is the node which is provided with a sensor section which detects physical data, a signal processing section which executes analog-to-digital conversion, calculation processing of the physical data, a radio section which transmits the processed data, and a power supply section which supplies power to each of these sections (for example, refer to Patent Literature 1).
As the sensor section, various types of sections are used according to the application. For example, an infrared sensor is extensively used in an information security field and a remote sensing field. The infrared sensors can be divided into a quantum type infrared sensor and a thermal type infrared sensor. The quantum type infrared sensor has a high efficiency in the sensitivity and the speed of response and so on, but generally needs the cooling and a high cost. Therefore, the quantum type infrared sensor is not too much used for a consumer use.
On the other hand, the thermal type infrared sensor does not need the cooling and is a comparatively low cost, because the sensor detects heat generation through absorption of infrared rays. As the thermal type infrared sensor, a pyroelectric type infrared sensor which detects heat generation due to the infrared rays by using the pyroelectric effect of ferroelectric substance, a bolometric type infrared sensor which detects heat generation due to the resistance change, and a thermopile type infrared sensor which detects thermoelectromotive force by using the thermoelectric effect and so on are exemplified.
A power supply section is often composed of primary battery. However, recently, a method of using radio power supply and a method of collecting power through the environment generation are used. As a method of the environment generation, photoelectric conversion using solar photovoltaic power generation, and vibration conversion which converts vibration into electricity by a piezoelectric element, thermoelectric conversion which converts heat into electrical energy and so on are used.
In the signal processing section, the analog-digital conversion of the obtained raw data, the integration, organization and conversion of the obtained data, and a control for radio communication and so on are mainly carried out. The signal processing section codes data for the radio communication according to the necessity. The signal processing section does not need a large-scale and high-speed processing unlike the personal computer. In the present situation, as the signal processing section, a comparatively small-scale microcomputer is used. The processing ability of the microcomputer is about 8 bits and the clock frequency is several MHz to tens of MHz. In this case, because the long life operation is important, a small power consumption amount than the conventional processor is strongly required. Also, although being different from the other information processing system, in the sensor, a standby time during which measurement and communication are carried out becomes long. Therefore, a sleep mode is also important in which a low power operation or an operation stop is carried out while an internal state is maintained.
The radio section communicates through use of a microwave oscillator and a receiver. However, high capacity communication and high power oscillation are not always needed unlike a mobile phone and a wireless LAN. As the radio communication method, Zigbee (frequency: the 2.4 GHz band, transmission rate: 250 kbps, maximum communication distance: 30 m) and so on are often used which is for a short distance communication and has a comparatively low power consumption amount. However, recently, further power saving method is proposed and studied.
Also, as a new technique which can contribute to the above ubiquitous sensor network, spintronics is remarkable in recent years. The spintronics is a technique that the degree of freedom of the spin is newly utilized effectively in addition to the degree of freedom of the electric charge, and is expected in a low power consumption amount of information processing and application to a new energy conversion. For example, an electric current is necessarily accompanied by the joule heat generation. On the other hand, the spin current which is a flow of spins, can realize information communication in a very small energy loss from the physical property (for example, refer to Patent Literature 2). Also, the spin current can be flow even in insulator in which electrons are restrained, and completely new utilization is expected for the information processing and the sensing (for example, Non-Patent Literature 1, and Non-Patent Literature 2).
Some techniques which convert the spin current into another physical quantity are known. For example, a technique which uses a spin Hall effect is known as the technique which converts an electric current into the spin current. The spin Hall effect has the effect that when applying an electric current into material, the spin current is generated in a direction orthogonal to a direction of the electric current (for example, refer to Non-Patent Literature 3). Oppositely, a reverse spin Hall effect is found in which the electric current is generated in a direction perpendicular to a direction of the spin current (for example, refer to Non-Patent Literature 4).
Also, as a technique which generates the spin current from a spatial temperature gradient, a spin Seebeck effect has been reported. The spin current generated by the heat can be taken out as the electric current through the reverse spin Hall effect (for example, refer to Patent Literature 3).